Scalaz Task not starting - scala

Question

I'm trying to make an asyncronous call using scalaz Task.
For some strange reason whenever I attempt to call methodA, although I get a scalaz.concurrent.Task returned I never see the 2nd print statement which leads me to believe that the asynchronous call is never returned. Is there something that I'm missing that is required in order for me to start the task?
for {
foo <- println("we are in this for loop!").asAsyncSuccess()
authenticatedUser <- methodA(parameterA)
foo2 <- println("we are out of this this call!").asAsyncSuccess()
} yield {
authenticatedUser
}
def methodA(parameterA: SomeType): Task[User]

Answer 1

First off, Scalaz Task is lazy: it won't start when you create it or get from some method. It's by design and allows Tasks to combine well. It's not intended to start until you fully assemble your entire program into single main Task, so when you're done, you need to manually start an aggregated Task using one of the perform methods, like unsafePerformSync that will wait for the result of aggregate async computation on a current thread:
(for {
foo <- println("we are in this for loop!").asAsyncSuccess()
authenticatedUser <- methodA(parameterA)
foo2 <- println("we are out of this this call!").asAsyncSuccess()
} yield {
authenticatedUser
}).unsafePerformSync
As you can see from your original code, you've never started any Task. However, you've mentioned that first println printed its message to console. This may be related to asAsyncSuccess method: I've not found it in Scalaz, so I assume it's in implicit class somewhere in your code:
implicit class TaskOps[A](val a: A) extends AnyVal {
def asAsyncSuccess(): Task[A] = Task(a)
}
If you write a helper implicit class like this, it won't convert an effectful expression into lazy Task action, despite the fact that Task.apply
is call-by-name. It will only convert its result, which happens to be just Unit, because its own parameter a is not call-by-name. To work as intended, it needs to be like the following:
implicit class TaskOps[A](a: => A) {
def asAsyncSuccess(): Task[A] = Task(a)
}
You may also ask why first println prints something while second isn't. This is related to for-comprehensions desugaring into method calls. Specifically, compiler turns your code into this:
println("we are in this for loop!").asAsyncSuccess().flatMap(foo =>
methodA(parameterA).flatMap(authenticatedUser =>
println("we are out of this this call!").asAsyncSuccess().map(foo2 =>
authenticatedUser)))
If asAsyncSuccess doesn't respect intended Task semantics, first println gets executed immediately. But its result is then wrapped into Task that is, essentially, just a wrapper over start function, and flatMap implementation on it just combines such functions together without running them. So, methodA or second println won't be executed until you call unsafePerformSync or other perform helper method. Note, however, that second println will still be executed earlier than it should according to Task semantics, just not that earlier.